This invention relates to fiber-reinforced resins, more particularly to composite materials containing layers of fiber-reinforced plastics and one or more layers of non-reinforced plastics.
It is known that the incorporation of reinforcing fibers into resinous articles significantly enhances the mechanical properties thereof. Thus, fiber-reinforced plastics are commonly employed in applications requiring high strength and stiffness. Since, however, the inclusion of such fibers adds both to the density and the cost of the composite, it is often desirable to provide a means by which the fiber content can be reduced while retaining good mechanical properties.
In U.S. Pat. No. 4,431,696 it is taught to prepare a "sandwich" structure having fiber-reinforced skins thermowelded to a plastic core. In this patent, it is necessary that the polymers of the skins be compatible with those of the core, since the bonding of the respective layers together is necessarily done by thermowelding. This greatly reduces the types of polymers suitable for use in the disclosed structures. In fact, it is often desirable to use different polymers in the core and outer layers of the composite, to take advantage of particularly beneficial combinations of properties. In addition, the outer glass-reinforced layers of the composite of the '696 patent are prepared from a fibrillar polymer, which greatly reduces the types of polymers available for use in such composites as well as significantly increasing the cost of the composite. The use of such fibrillar polymers renders the manufacture and processing of the outer layers more difficult and costly. It is also difficult to obtain good physical properties using such fibrillar polymers.
Thus, it would be desirable to provide a high strength fiber-reinforced resin having relatively low levels of reinforced fibers, but in which a wide range of resins are useful.